In a typical cross-fin type heat exchanger in which a plurality of heat transfer fins are arranged in an array around straight pipe portions of a serpentine heat transfer tube with a plurality of bends, when heat transfer surfaces are cooled so that the surface temperature is at or below the air dew point temperature, condensation of water vapor in the air occurs on the heat transfer surfaces and water droplets are generated on the surfaces. In particular, when the temperature of the fins is at or below 0° C., a frosting phenomenon occurs such that water vapor in the air forms frost on the heat transfer surfaces. As the frost on the heat transfer surfaces grows, air paths through which the air passes are clogged. Disadvantageously, airflow resistance increases, so that the performance of an apparatus markedly decreases.
To avoid the performance decrease due to frost, a defrosting operation for removing frost formed on the surfaces of the heat exchanger has to be periodically performed. For the defrosting operation, for example, a hot gas system in which the heat exchanger, serving as a target, is heated from the inside by switching of flow directions of a refrigerant in a refrigeration cycle or a heater system in which the heat exchanger is heated from the outside by a heater disposed near the heat exchanger is used. During the defrosting operation, a role of the apparatus, for example, comfort of air conditioning, is reduced. Furthermore, the efficiency of such a device is also reduced. It is therefore necessary to shorten the time of the defrosting operation as much as possible.
As regards the frost problem, according to a related-art, the surface of each fin is coated with a hydrophilic coating layer, the hydrophilic coating layer is exposed to plasma to form fine asperities thereon so that the area of the hydrophilic coating layer on the surface of the fin is increased, thus enhancing the effect of the coating layer, namely, providing superhydrophilicity. Accordingly, adhesion water, which will cause frost, becomes to have affinity with the surface of the fin, thus facilitating gravitational flow discharge. Alternatively, the surface of each fin is coated with a water-repellent or hydrophobic coating layer, the hydrophobic coating layer is exposed to plasma to form fine asperities so that the area of the hydrophobic coating layer on the surface of the fin is increased, thus enhancing the effect of the coating layer, namely, providing superhydrophilicity. Accordingly, adhesion water, which will cause frost, tends to be shaped into a sphere, thus facilitating gravitational flow discharge from the surface of the fin. Consequently, forming of frost is delayed (refer to Patent Literature 1, for example).